A crystallized silicon film obtained by crystallizing amorphous silicones is used as a channel of a thin film transistor (TFT) for driving pixels of a flat panel display (FPD), a channel of a memory cell transistor of a non-volatile semiconductor memory, or the like.
The reason why the amorphous silicones are crystallized is to improve carrier mobility. The carrier mobility increases in order of amorphous silicon→polycrystalline silicon→single crystalline silicon. As such, in a film forming process, silicones of a silicon film being in an amorphous state is first crystallized to use as the channel of the transistor, for example. There are known various methods of crystallizing the amorphous silicones.
A first conventional method includes depositing amorphous silicones on an insulating film in two stages. At this time, a deposition temperature in the first stage is set to be higher than that in the second stage. Further, a hydrogen doping process is performed in the first stage. This suppresses an increase in density of crystal seeds so that an SOI (silicon-on-insulator) substrate having large grain size is manufactured.
A second conventional method includes forming a stacked structure of a non-doped amorphous silicon film followed by a phosphorus (P)-doped amorphous silicon film on a substrate, and heating the stacked structure at 600 degrees C. This method is based on the fact that a grain size increases with increases in the doping amount of the phosphorus. Initially, amorphous silicones of the phosphorus-doped amorphous silicon film are crystallized and subsequently, amorphous silicones of the non-doped amorphous silicon film are crystallized using the crystallized amorphous silicones as crystal seeds. Thus, a polycrystalline silicon film having large grain size is obtained.
However, the first conventional method requires switching the deposition temperature in the deposition of the amorphous silicones. To do this, a period of time is required to increase or decrease the temperature, which results in a degradation of productivity.
Further, in the second conventional method, a phosphorus (P) doping process is performed. Phosphorus is a chemical element of V group and serves to change a chemical element of IV group into an N-type semiconductor. When the doping amount of phosphorus is increased to increase a grain size, an N-type polycrystalline silicon film is obtained. If the N-type polycrystalline silicon film is used as the channel of the transistor, only a P-channel transistor is formed. To obtain an N-channel transistor requires changing the N-type polycrystalline silicon film into a P-type polycrystalline silicon film. In order to change the N-type polycrystalline silicon film, in which a large amount of phosphorus is diffused, into the P-type polycrystalline silicon film, it is necessary to introduce a large amount of a chemical element of III group. However, such an introduction is not practical in terms of productivity. In addition, the introduction may cause a crystal defect.